Effects of plant size and morphological constraints on variation in reproductive components in two related species of Epilobium

1 This study explored the effects of plant size, position of fruits within inflorescences, and position of seeds within fruits on the regulation of fecundity in the two outcrossing perennial herbs Epilobium dodonaei and E. fleischeri which replace each other along an altitudinal gradient. 2 Almost all reproductive components were affected by size. The largest difference between small and large individuals of both species concerned the number of mature seeds per stem, followed by the number of fruits per stem, flower buds per stem, seeds per fruit, ovules per fruit and individual seed mass. 3 The increase in the number of seeds per stem from small to large plants was not only due to the increasing number of flower buds per stem and ovules per fruit, but also a result of a decreasing abortion rate of flower buds and seeds. 4 The fruit/flower ratio was not affected by plant size, but the fruit/flower-bud ratio increased from 53% in small plants to 68% in large plants of E. fleischeri and from 22 to 62% in E. dodonaei, respectively. Early abortion of flower buds determined the abortion pattern, abortions after flowering were rare. The pattern of abortion in both species was strongly affected by the position of flower buds within the inflorescences. Buds from intermediate parts of the inflorescence had the smallest abortion frequency. 5 Seed/ovule ratio was 50% in E. fleischeri and 40% in E. dodonaei. The fate of ovules was not affected significantly by the size of plants. However, positional effects of fruits within inflorescences and of ovules within fruits on abortions were highly significant. 6 The overwhelming part of the variation in individual seed mass in both species was found within fruits and among fruits within a single inflorescence. Only in E. fleischeri was individual seed mass affected by the size of plants. 7 Our results suggest that abortion patterns in reproductive components are strongly affected by changes in resource availability during plant development, but also by morphological constraints. 8 Although the alpine E. fleischeri produced fewer fruits than E. dodonaei, a trade-off between seed number and seed mass contributed to a similar seed number per shoot in both species. We conclude that the pattern and regulation capacity in reproductive components are tightly linked to specific life-history traits of the two species.